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      <h2><a href="./">Go by Example 中文版</a>: Switch 分支结构</h2>
      
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            <p><em>switch</em> 是多分支情况时快捷的条件语句。</p>

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            <a href="http://play.golang.org/p/nfMythkmw1D"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
          <div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>

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          <div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
    <span class="s">&quot;fmt&quot;</span>
    <span class="s">&quot;time&quot;</span>
<span class="p">)</span>
</pre></div>

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          <div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>

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            <p>一个基本的 <code>switch</code>。</p>

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          <div class="highlight"><pre>    <span class="nx">i</span> <span class="o">:=</span> <span class="mi">2</span>
    <span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="s">&quot;write &quot;</span><span class="p">,</span> <span class="nx">i</span><span class="p">,</span> <span class="s">&quot; as &quot;</span><span class="p">)</span>
    <span class="k">switch</span> <span class="nx">i</span> <span class="p">{</span>
    <span class="k">case</span> <span class="mi">1</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;one&quot;</span><span class="p">)</span>
    <span class="k">case</span> <span class="mi">2</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;two&quot;</span><span class="p">)</span>
    <span class="k">case</span> <span class="mi">3</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;three&quot;</span><span class="p">)</span>
    <span class="p">}</span>
</pre></div>

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            <p>在同一个 <code>case</code> 语句中，你可以使用逗号来分隔多个表达式。
在这个例子中，我们还使用了可选的 <code>default</code> 分支。</p>

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          <div class="highlight"><pre>    <span class="k">switch</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Now</span><span class="p">().</span><span class="nx">Weekday</span><span class="p">()</span> <span class="p">{</span>
    <span class="k">case</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Saturday</span><span class="p">,</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Sunday</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s the weekend&quot;</span><span class="p">)</span>
    <span class="k">default</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s a weekday&quot;</span><span class="p">)</span>
    <span class="p">}</span>
</pre></div>

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            <p>不带表达式的 <code>switch</code> 是实现 if/else 逻辑的另一种方式。
这里还展示了 <code>case</code> 表达式也可以不使用常量。</p>

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          <div class="highlight"><pre>    <span class="nx">t</span> <span class="o">:=</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Now</span><span class="p">()</span>
    <span class="k">switch</span> <span class="p">{</span>
    <span class="k">case</span> <span class="nx">t</span><span class="p">.</span><span class="nx">Hour</span><span class="p">()</span> <span class="p">&lt;</span> <span class="mi">12</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s before noon&quot;</span><span class="p">)</span>
    <span class="k">default</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s after noon&quot;</span><span class="p">)</span>
    <span class="p">}</span>
</pre></div>

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            <p>类型开关 (<code>type switch</code>) 比较类型而非值。可以用来发现一个接口值的类型。
在这个例子中，变量 <code>t</code> 在每个分支中会有相应的类型。</p>

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          <div class="highlight"><pre>    <span class="nx">whatAmI</span> <span class="o">:=</span> <span class="kd">func</span><span class="p">(</span><span class="nx">i</span> <span class="kd">interface</span><span class="p">{})</span> <span class="p">{</span>
        <span class="k">switch</span> <span class="nx">t</span> <span class="o">:=</span> <span class="nx">i</span><span class="p">.(</span><span class="kd">type</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">case</span> <span class="kt">bool</span><span class="p">:</span>
            <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;I&#39;m a bool&quot;</span><span class="p">)</span>
        <span class="k">case</span> <span class="kt">int</span><span class="p">:</span>
            <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;I&#39;m an int&quot;</span><span class="p">)</span>
        <span class="k">default</span><span class="p">:</span>
            <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;Don&#39;t know type %T\n&quot;</span><span class="p">,</span> <span class="nx">t</span><span class="p">)</span>
        <span class="p">}</span>
    <span class="p">}</span>
    <span class="nx">whatAmI</span><span class="p">(</span><span class="kc">true</span><span class="p">)</span>
    <span class="nx">whatAmI</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
    <span class="nx">whatAmI</span><span class="p">(</span><span class="s">&quot;hey&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>

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          <div class="highlight"><pre><span class="gp">$</span> go run switch.go
<span class="go">Write 2 as two</span>
<span class="go">It&#39;s a weekday</span>
<span class="go">It&#39;s after noon</span>
<span class="go">I&#39;m a bool</span>
<span class="go">I&#39;m an int</span>
<span class="go">Don&#39;t know type string</span>
</pre></div>

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        下一个例子: <a href="arrays">数组</a>
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